Deoxys, the Space Virus Pokémon introduced in Ruby & Sapphire has 4 forms spread across the games. However in the past, you had to trade your Deoxys across the games, this is no more.

In Veilstone City, you have the ability to change your Deoxys form. In the east of the city there are 4 Meteorites. When you have Deoxys with you in your party, talk to them and they will change your Deoxy's Form.

Top - Speed Deoxys

Middle - Normal Deoxys

Bottom Left - Attack Deoxys

Bottom Right - Defense Deoxys

All Deoxys forms have different stats and learnsets so this could be handy if you want a particular form of Deoxys that knows a certain attack